Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Touch input devices or sensors for computers and other consumer electronics devices such as mobile phones, personal digital assistants (PDAs) and hand-held games are highly desirable due to their relative ease of use. In the past, a variety of approaches have been used to provide touch input devices. The most common approach uses a flexible resistive overlay, although the overlay is easily damaged, can cause glare problems, and tends to dim the underlying screen, requiring excess power usage to compensate for such dimming. Resistive devices can also be sensitive to humidity, and the cost of the resistive overlay scales quadratically with perimeter. Another approach is the capacitive touch screen, which also requires an overlay. In this case the overlay is generally more durable, but the glare and dimming problems remain.
In yet another common approach, a matrix of infrared light beams is established in front of a display, with a touch detected by the interruption of one or more of the beams. Such optical touch input devices have long been known (see U.S. Pat. No. 3,478,220 and U.S. Pat. No. 3,673,327), with the beams generated by arrays of optical sources such as light emitting diodes (LEDs) and detected by corresponding arrays of detectors (such as phototransistors). They have the advantage of being overlay-free and can function in a variety of ambient light conditions (U.S. Pat. No. 4,988,983), but have a significant cost problem in that they require a large number of source and detector components, as well as supporting electronics. Since the spatial resolution of such systems depends on the number of sources and detectors, this component cost increases with display size and resolution. Usually, the optical sources and detectors oppose each other across the display, although in some cases (disclosed for example in U.S. Pat. No. 4,517,559, U.S. Pat. No. 4,837,430 and U.S. Pat. No. 6,597,508) they are located on the same side of the display, with the return optical path provided by a reflector on the opposite side of the display.
An alternative optical touch input technology, based on integrated optical waveguides, is disclosed in U.S. Pat. No. 6,351,260, U.S. Pat. No. 6,181,842 and U.S. Pat. No. 5,914,709. The basic principle of such a device is shown in FIG. 1. In this design, integrated optical waveguides 10 conduct light from an optical source 11 to integrated in-plane lenses 16 that collimate the light in the plane of a display and/or input area 13 and launch an array of light beams 12 across that display and/or input area 13. The light is collected by a second set of integrated in-plane lenses 16 and integrated optical waveguides 14 at the other side of the display and/or input area, and conducted to a position-sensitive (i.e. multi-element) detector 15. A touch event (e.g. by a finger or stylus) cuts one or more of the beams of light and is detected as a shadow, with position determined from the particular beam(s) blocked by the touching object. That is, the position of any physical blockage can be identified in each dimension, enabling user feedback to be entered into the device. Preferably, the device also includes external vertical collimating lenses (VCLs) 17 adjacent to the integrated in-plane lenses on each side of the input area, to collimate the light in the direction perpendicular to the plane of the input area.
As shown in FIG. 1, the touch input devices are usually two-dimensional and rectangular, with two arrays (X, Y) of transmit waveguides 10 along adjacent sides of the input area, and two corresponding arrays of receive waveguides 14 along the other two sides of the input area. As part of the transmit side, in one embodiment light from a single optical source 11 (such as an LED or a vertical cavity surface emitting laser (VCSEL)) is distributed to a plurality of transmit waveguides 10 forming the X and Y transmit arrays via some form of optical splitter 18, for example a 1×N tree splitter. The X and Y transmit waveguides are usually arranged on an L-shaped substrate 19, and the X and Y receive waveguides arranged on a similar L-shaped substrate, so that a single source and a single position-sensitive detector can be used to cover both X and Y dimensions. However in alternative embodiments, a separate source and/or detector may be used for each of the X and Y dimensions. Additionally, the waveguides may be protected from the environment by a bezel structure that is transparent at the wavelength of light used (at least in those portions through which the light beams 12 pass), and may incorporate additional lens features such as the abovementioned VCLs. Usually the sensing light is in the near IR, for example around 850 nm, in which case the bezel is preferably opaque to visible light.
For simplicity, only four pairs of transmit and receive waveguides per dimension are shown in FIG. 1. Generally there will be many more pairs per dimension, closely spaced so that the light beams 12 substantially cover the input area 13.
Compared to touch input devices with paired arrays of sources and detectors, waveguide-based devices have a significant cost advantage because of the greatly reduced number of optical sources and detectors required. Nevertheless, they still suffer from a number of drawbacks.
Firstly, because touch functionality is being increasingly common in consumer electronics devices such as mobile phones, handheld games and personal digital assistants (PDAs), there is a continuing requirement to reduce costs. Even if relatively inexpensive waveguide materials and fabrication techniques (such as curable polymers patterned by a photolithographic or moulding process) are used, the transmit and receive waveguide arrays still represent a significant fraction of the cost of the touch input device. Secondly there is a signal-to-noise problem: because the transmit waveguides are small (typically they have a square or rectangular cross section with sides of order 10 m), it is difficult to couple a large amount of signal light into them from the optical source. Since only a fraction of this light will be captured by the receive waveguides, the system as a whole is vulnerable to noise from ambient light, especially if used in bright sunlight. Thirdly, because the device uses discrete beams 12, the transmit and receive waveguides need to be carefully aligned during assembly. A similar alignment requirement applies to the older optical touch input devices with arrays of discrete sources and detectors.
Inspection of the waveguide-based touch input device shown in FIG. 1 reveals that positional information for a touching object is encoded on the receive waveguides 14; that is to say, the position of the object is determined from those particular receive waveguides that receive less or no light and convey that condition to the respective elements of the multi-element detector 15. The transmit side is less critical, and two sheets of light propagating in the X and Y directions can be used in place of the grid of discrete beams 12.
An alternative configuration disclosed in U.S. Pat. No. 7,099,553 and shown schematically in FIG. 2 provides a sheet of light, while still using a minimal number of optical sources, by replacing the transmit waveguides with a single bulk optics waveguide in the form of a light pipe 21 with a plurality of reflective facets 22. In operation, light from an optical source 11 is launched into an input face of the light pipe 21, optionally with the assistance of a lens 23, and this light is deflected by the reflective facets 22 to produce sheets of light 45 that traverse the input area 13 towards the receive waveguides 14. As shown in FIG. 2, the light pipe 21 is an L-shaped item encompassing both transmit sides of the input area 13, with a turning mirror 24 at its apex. In a minor variation there may be separate, substantially linear light pipes for each of the transmit sides. Advantageously, the light pipe 21 may comprise a polymer material formed by injection moulding for example, and as such will be considerably less expensive to fabricate than an array of waveguides. It will be further appreciated that since the light pipe 21 is a bulk optics component, it will be relatively straightforward to couple light into it with high efficiency from an optical source 11, thereby improving the signal-to-noise ratio.
As mentioned in U.S. Pat. No. 7,099,553, the output faces 25 of the light pipe 21 can be shaped with cylindrical curvature to form lenses 26 that collimate the light sheets 45 in the vertical (i.e. out-of-plane) direction, obviating the need for any separate vertical collimating lens. This will further reduce the Bill of Materials, and possibly also the assembly costs.
Light pipes with a plurality of reflective facets are commonly used for distributing light from a single light source for illumination purposes (see for example U.S. Pat. No. 4,068,121). Two-dimensional versions such as a substantially planar light guide plate with a plurality of reflective facets on one surface are also known for display backlighting, as disclosed in U.S. Pat. No. 5,050,946 for example. In most known light pipes and light guide plates, the reflective facets are formed along an exterior edge or surface. The light pipe 21 disclosed in U.S. Pat. No. 7,099,553 has a rather different form, where the facets 22 are essentially internal to the light pipe body, and are stepped in height so that each facet only reflects a small fraction of the light guided within the light pipe. An advantage with this design is that the width 27 of the light pipe is relatively small, which is important for touch input devices where the “bezel width” around a display should not be excessive. However it has the significant disadvantage of being a complicated design, with numerous sharp corners and concave portions that will be extremely difficult to reproduce accurately via injection moulding. A second problem is that, analogous to the well-known principle of single slit diffraction, the divergence angle of a light beam reflected off a facet will depend on the height of that facet. Therefore the incremental height of the facets 22 in the light pipe 21 will cause the reflected beams to have incrementally varying divergence in the out-of-plane direction, such that a simple cylindrical lens 26 will not be able to completely collimate the light sheets 45.
A much simpler optical touch input device where a minimal number of optical sources are used to generate a sheet of sensing light is disclosed in U.S. Pat. No. 4,986,662. As illustrated in FIG. 2A, a touch input device includes a rectangular frame 91 with an optical source 11 and an array of detectors 56 along two sides and parabolic reflectors 92 on the opposing two sides. Light 35 emitted from each optical source propagates across the input area 13 towards a respective parabolic reflector, and is reflected back across the input area as sheets of light 45 in the X and Y dimensions. Unfortunately this simple configuration has the disadvantage that in many parts of the input area, a touch object 60 will block the outgoing light 35, complicating the detection algorithms.
The present disclosure overcomes or ameliorates at least one of the disadvantages of the prior art, or provides a useful alternative.